DuckDuckGo uses Wordnick, which seems like a pretty good option. It pulls from several public-domain dictionary sources, like Wiktionary, WordNet and the 1911 Century Dictionary. It's a nonprofit, shows multiple definitions and has an API built for search (since ddg uses it). When compared to wordsAPI, it has much more definitions, as well as linked definitions inside the definition, which can be clicked to learn even more words. It's a good middle ground between the expensive Oxford English Dictionary and wordsapi.
Ever since Google has replaced Oxford definitions with AI summary definitions, Kagi has had a great opportunity to attract linguists, academics, lexicologists, students, etc.
